The present invention relates to locating a position of an object implanted in a portion of the anatomy, the object having at least a portion which is in a vicinity of a material having a different acoustic impedance than an acoustic impedance of that portion of the object. More particularly, the present invention relates to the localization of implanted targets using amplitude-mode (A-mode) ultrasound techniques and a coordinate space digitizer. Extendable to other applications, the automatic implanted target localization approach may be used to locate small fluid-filled polymer cylinders implanted in human skull, which are preferably flush with the surface of the skull, beneath the scalp and subcutaneous fat. These permanently-implanted cylinders are intended to serve as fiducial markers for the registration of tomographic image volumes with physical space in neurosurgical cases, as well as for tracking a patient over time.
Tomographic imaging modalities, such as computer tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET), produce a three-dimensional image volume as a set of three-dimensional "slices". These image volumes contain the information necessary for surgical or radiotherapeutic planning. Such information may include the three-dimensional distances between sites of interest such as tumors, the location of blood vessels which must be avoided, and lesion margin delineation often superior to that discernible by visual tissue inspection.
However, these three-dimensional volumes are at an arbitrary orientation to each other as well as to the physical patient anatomy. As a result, correlating and comparing the same location in the images is difficult. Also, the surgeon is unable to accurately correlate the detailed image information with the physical anatomy during surgery as a guidance tool.
The correlation of multiple three-dimensional volumes or spaces is referred to as space registration. Space registration establishes a one-to-one mapping between points in the image sets or between points in one or more image sets and physical space. The transformation between coordinate systems is calculated based on the location of a set of at least three common landmarks, or fiducial markers, in each representation. The actual fiducial point is the geometric center of the fiducial marker. The accuracy of the mapping between coordinate systems depends on the accuracy with which the coordinates of the fiducial markers centers are known in each three-dimensional space.
The fiducial markers provide a frame of reference to make image-to-image or image-to-physical space registration possible. A general technique for using fiducial markers to obtain registration of image data across time is set forth in U.S. Pat. No. 4,991,579 to Allen et al., which is incorporated herein by reference.
Fiducial markers for accurate image-to-physical space registration must be rigidly located and must be composed of materials making them visible in the imaging modalities of interest. U.S. patent application Ser. No. 08/017,167 to McCrory et al., which is incorporated herein by reference, describes the design and composition of fiducial markers for neurosurgical image registration and image-physical space registration, as well as a method for localizing the center of the imaged markers in the image volumes. This patent application describes two types of markers including temporary markers anchored in the skull for rigidity but with the image-visible portion protruding above the scalp, and permanent markers implanted into the skull beneath the scalp and subcutaneous fat.
Internal permanent fiducial markers allow the comparison of images over time for follow-up therapy. Permanent markers also allow the delivery of fractionated radiotherapy, in which small doses of radiation are administered in multiple sessions to maximize the dose delivered to the target lesion while minimizing the damage to surrounding tissue. Fractionated radiotherapy requires the same fiducial framework to be present, in an unchanged position, for each treatment so that the radiation beam may be properly directed relative to the fiducial markers as determined from the pre-treatment images. Temporary markers and stereotactic frames can neither remain in position long enough nor be re-affixed accurately to satisfy this requirement.
In addition to a fiducial marking approach, image-to-physical space registration requires a method for establishing the coordinate system in physical space. Several coordinate space digitizers including specific pointing devices have been developed which define a coordinate space and pass the three-dimensional coordinates of the endpoint to an attached computer. As an example, an articulated arm has joints that track the angular position of each link, allowing the attached computer to calculate the endpoint coordinates. A less cumbersome alternative is a wand with infrared light emitting diodes (LEDs) along its shaft in which the LEDs are strobed in sequence and an infrared camera attached to a computer notes the location of the LEDs relative to a set of reference LEDs in a fixed position.
Also required of a system which correlates neurological image space and physical space is a means for locating the center of the fiducial markers in the coordinate system defined by the pointing device. For markers visible outside the scalp, the fiducial position can be recorded simply by touching the fiducial marker with the pointing device. For permanent, subcutaneously-implanted markers, however, locating the precise three-dimensional position of the marker is much more challenging. A target localization method to address this task has previously become necessary in the field of registration of image volumes with physical space in neurosurgical cases.
Once the markers have been located in the preoperative image sets stored on a computer as well as in the physical space defined by a pointing device attached to the same computer, the system display can provide interactive surgical guidance. The location of the endpoint of the pointing device is indicated on a display of the appropriate slice through the image volumes. Such an interactive, image-guided surgery system using an articulated arm as a pointing device is described in U.S. Pat. No. 5,142,930 to Allen et al., which is incorporated herein by reference.
U.S. Pat. No. 5,197,476 to Nowacki et al., which is also incorporated herein by reference, discloses an ultrasound probe coupled with an infrared LED-based pointing device to locate a target in a living body. A three-dimensional frame containing a plurality of infrared lights is placed on a table. A computer strobes the infrared lights and the position of the infrared lights is monitored by a pair of infrared sensitive cameras and stored in the computer. A hand held ultrasonic probe is provided with a plurality of infrared lights so that the probe can be monitored by the cameras. The computer compares the positions of the probe lights with the initial position of the frame infrared lights to accurately determine the position of the probe so that the position of the target in the body may be displayed on a computer monitor. The approach disclosed in the Nowacki et al. patent employs a brightness-mode (B-mode) ultrasound imaging and requires a trained expert to visually recognize when the desired target is displayed.